Recently, electrically-powered vehicles have been of interest as environmentally-friendly vehicles. The electrically-powered vehicles are each equipped with a power storage device (such as secondary battery or capacitor for example) and travel using a drive force generated from electric power stored in the power storage device. The electrically-powered vehicles include for example electric vehicles, hybrid vehicles, fuel cell vehicles, and the like.
Some of these electrically powered vehicles have a motor generator receiving electric power from the power storage device to generate a drive force for the vehicle to travel when started or accelerated, and generating electric power by means of regenerative braking when the vehicle is braked to store the generated electrical energy in the power storage device. Thus, in order to control the motor generator as appropriate for the traveling condition, the electrically powered vehicles are each equipped with an inverter.
Some of such vehicles have a voltage converter (converter) between the power storage device and the inverter for stably supplying electric power which is required by the inverter and varies depending on the vehicle condition. This converter can be used to provide an input voltage of the inverter higher than an output voltage of the power storage device so that a high motor output is achieved, and reduce the motor current for the same output so that smaller sizes of the inverter and the motor as well as a lower cost are achieved.
In order to further improve the fuel economy of the electrically powered vehicles, it is important to reduce a loss of this converter and thereby improve the efficiency.
WO2003/015254 (PTL 1) discloses a motor drive control device converting an output voltage from a DC power supply by means of a converter to supply the converted voltage to an inverter which drives a motor. When a target input voltage value of the inverter is lower than a power supply voltage of the DC power supply, the motor drive control device renders an upper switching element of the converter ON and renders a lower switching element thereof OFF.
When the motor operates at a low rotational speed where a target input voltage value of the inverter is lower than a power supply voltage of the DC power supply, this motor drive control device can reduce a switching loss by stopping the operation of the switching element and thereby improve the efficiency of the converter.